This invention relates to non-destructive methods and apparatus for evaluating strain on a workpiece surface. The invention further relates to polarization-dependent reflective devices.
It is often desirable not only to measure the magnitude of an applied strain, but to know the tensor and even the precise location of the strain in the workpiece. Prior art strain gauges have been unable to provide this information in a simple and low-cost manner.
One type of fiber-optic strain gauge relies upon an optical fiber which transmits light along its length with little or no loss when straight. When deformed under an applied stress, i.e., by bending, significant attenuation of the light signal occurs. The extent of light transmission loss may be correlated to the deformation strain. J. D. Weiss in xe2x80x9cFiber-Optic Strain Gaugexe2x80x9d (J. Lightwave Tech. 7(9), September 1989) used the same principle to design an fiber-optic strain gauge using optical fibers having permanent microbends which demonstrate an increase in light transmission under applied tension. Fiber-optic strain gauges are able to identify the existence of strain, but are unable to identify its location within the article since bending (or extension of the microbend) is propagated along the entire length of the optical fiber and is not restricted to the strain site.
Fiber-optic Bragg gratings have also been described for measuring strain. Bragg gratings on an optical fiber are produced by UV exposure causing periodic changes in the index of refraction of the fiber core. A typical Bragg grating strain sensing system involves Bragg wavelength shift detection. Such sensing systems have very low reflectivity, making the sensitivity of the measurements low. Furthermore, the fragility of the optical fibers gives rise to the need to embed the fibers in a matrix so that only bulk strain is practically measured using this technique.
Photolithography has been used to form diffraction gratings in metallic strips. Applied stress causes the grating dimensions to shift which can be monitored to detect strain. The preparation and use of photolithographic diffraction gratings is awkward and computationally complex. Furthermore, the diffraction grating can not provide information regarding the location of strain in a workpiece.
Strain gauges have been described which rely upon the optical properties of thin films to detect and even quantify strain. U.S. Pat. No. 4,123,158 reports on the use of optical properties such as birefringence to detect surface strain. Photoelastic materials are used which have the optical property of polarizing light when under stress and then transmitting such light on the principle stress planes with velocities depending upon the magnitude of the applied stress. When such gauges are subjected to monochromatic polarized light, the birefringence of the photoelastic material causes the light to emerge refracted in two orthogonal planes. Because the refractive indices of light propagation are different in each direction, a phase shifting of the light waves occurs. When the waves are combined with polarizing film, regions of stress where the wave phase is canceled appear black and regions of stress where the wave phase is combined appear white. When white light is used in place of monochromatic light, the relative retardation of the photoelastic material causes the fringes to appear in colors of the spectrum.
Reflective liquid crystal displays have been developed which rely on polymer dispersed liquid crystals (PDLCs). A conventional PDLC is formed by phase separation of a liquid crystal phase from a matrix polymer phase. Photopolymerization-induced phase separation utilizes a mixture of a low molecular weight liquid crystal and a photocurable monomer. Irradiation of the homogeneous pre-polymer mixture initiates polymerization, which in turn induces a phase separation between the polymer and liquid crystal (LC). Liquid crystal droplets are formed within the sample to modulate the LC droplet density on the order of the wavelength of light.
H-PDLCs are phase separated compositions formed under holographic conditions. Holographic or optical interference preparative techniques have been used to carry out polymerization to selectively position regions of liquid crystal and polymer. Instead of random arrangement of LC droplets, the holographic exposure induces a periodic array of LC droplets and matrix polymer planes. On exposure to an optical interference pattern, typically formed by two coherent, counter-propagating lasers, polymerization is initiated in the light fringes. A monomer diffusion gradient is established as the monomer is depleted in the dark fringes, causing migration of liquid crystal from the dark fringes. The result is LC-rich areas where the dark fringes were located and essentially pure polymer regions where the light fringes were located.
Thin H-PDLC films have been incorporated into displays relying upon the optical reflective properties of the material to provide visual images. Displays incorporating these materials have been reported in xe2x80x9cHolographically formed liquid crystal/polymer device for reflective color displaysxe2x80x9d by Tanaka et al. in Journal of the Society for Informational Display (SID), Volume 2, No. 1, 1994, pages 37-40; and also in xe2x80x9cOptimization of Holographic PDLC of Reflective Color Display Applicationsxe2x80x9d in SID ""95 Digest, pages 267-270 (1995). The reflective properties of such films have not been exploited in the field of strain determination.
It is an object of the invention to provide an improved strain gauge capable of providing information regarding the existence, location and/or magnitude of strain in a workpiece.
It is a further object of the invention to provide a non-destructive strain gauge.
It is a further object of the invention to provide holographically-formed polymer dispersed liquid crystals (H-PDLCs) based devices for use in observation and measurement of strain.
It is a further object of the invention to provide polarization-sensitive reflective devices useful as strain gauges, displays or polarizing filters, and the like.
In one aspect of the invention, a reflective strain gauge includes an holographically-formed polymer dispersed liquid crystal (H-PDLC) film comprising layers of liquid crystal (LC) droplets in a polymer matrix, in which the H-PDLC film has a reflection or transmission grating capable of reflecting or transmitting light of a selected wavelength, and means for securing the film to a surface of a workpiece for monitoring the strain at said surface. Strain is observed by a change in the nature of the light reflected or transmitted from the surface of an H-PDLC-containing film.
In preferred embodiments, the film includes multiple gratings, wherein different gratings are responsive to stress applied in different directions. The multiple gratings are located within a single H-PDLC layer, or in a plurality of H-PDLC layers in which each layer has at least one grating. The grating is oriented within the H-PDLC film so that surface strain is observed as a blue shift or a red shift of the reflected light. In other embodiments, the intensity of the reflected light is polarization dependent. The LC layers may be substantially parallel, substantially perpendicular, or at an angle to the film surface.
In another preferred embodiment, the LC droplets of the H-PDLC film are oriented such that the refractive index parallel to the axis (ne) is greater than the refractive index perpendicular to the axis (no). no substantially matches the refractive index of the matrix polymer, so that light polarized perpendicular to the axis is transmitted, and light polarized parallel to the axis is reflected. Orientation may arise in the strained state such that such that in the LC droplets form ellipsoids with long axes aligned parallel to an axis of an applied force. Orientation may arise by H-PDLC formation under an orienting force.
In other embodiments, the matrix polymer is selected to have sufficient elasticity to sustain an applied strain without failure. The strain is proportionate to a strain of a workpiece.
In another embodiment of the invention, the film includes aspected particles embedded in an elastic polymer, and the aspected particles contain an H-PDLC material comprising layer of LC droplets in a matrix polymer. The aspected particles have an aspect ratio in the range of at least 4:1, and preferably are in the range of at least 10:1.
In another aspect of the invention, a method for detecting strain in an article is provided. The method includes attaching a reflective strain gauge to a surface of an article, the strain gauge comprising an holographically-formed polymer dispersed liquid crystal (H-PDLC) film having layers of liquid crystal (LC) droplets in a matrix polymer and having a reflection or transmission grating capable of reflecting or transmitting light of a selected wavelength, illuminating the film with light, and monitoring for a change in the reflected or transmitted light, which is associated with strain in the article.
In some embodiments, the change in the reflected light comprises a change in the wavelength of the reflected light, and the wavelength shift may be in the range of 5-100 nm, or is in the range of 5-50 nm, or in the range of about 10-25 nm.
In other embodiments, the change in the reflected light comprises a change in the intensity of the reflected light.
In other embodiments, the strain is the result of a compressive force, or of a tensile force, or of a torsional or shearing force. The film may be positioned such that when a tensile force is applied, the spacing between the layers contracts. The tensile force may be applied along the long axis of the LC droplet layers, and the shift may be a blue shift of the reflected or transmitted light. The film may be positioned such that when a tensile force is applied, the spacing between the layers expands. The tensile force is applied along a direction transverse to the long axis of the LC droplets layers, and the shift is a red shift of the reflected light.
In other embodiments, the method includes the step of illuminating the film with polarized light. In the strained state the LC droplets form ellipsoids with long axes aligned parallel to an axis of an applied force, such that the refractive index parallel to the axis (ne) is greater than the refractive index perpendicular to the axis (no). no substantially matches the refractive index of the polymer, so that light polarized perpendicular to the axis is transmitted, and light polarized parallel to the axis is reflected.
In other embodiments, the matrix polymer is selected to have sufficient elasticity to sustain strain without failure, the strain being proportional to the strain of the article. The film includes multiple reflection gratings, wherein different gratings are responsive to stress applied in different directions. The multiple gratings are located within a single H-PDLC layer, or in a plurality of H-PDLC layers and each layer includes at least one grating. The LC layers may be substantially parallel, substantially perpendicular, or at an angle to the article surface.
In other embodiments, the method includes the step of monitoring the wavelength shift by a technique selected from the group consisting of visual observation, photodiode observation and spectrophotometry. The applied strain may be in the range of up to about 21%, or greater dependent upon the materials selected for the H-PDLC matrix polymer.
In another embodiment of the invention, the film includes aspected particles embedded in an elastic polymer, and the aspected particles include an H-PDLC material comprising layer of LC droplets in a matrix polymer. The aspected particles orient along a direction of an applied force when the film is stressed.
In another aspect of the invention, a polarization-sensitive reflective display is provided having an holographically-formed polymer dispersed liquid crystal (H-PDLC) film comprising layers of liquid crystal (LC) droplets in a matrix polymer, the H-PDLC film having a reflection grating capable of reflecting light of a selected wavelength, wherein the reflection grating of the H-PDLC film is oriented, such that the refractive index parallel to the axis of orientation (xcex7e) is greater than the refractive index perpendicular to the axis (xcex7o) In some embodiments, the display has no substantially matches the refractive index of the matrix polymer, so that light polarized perpendicular to the axis is transmitted, and light polarized parallel to the axis is reflected.
In some embodiments of the display, orientation is attained by application of a strain, such that in the strained state the LC droplets form ellipsoids with long axes aligned parallel to the axis of the applied strain. In other embodiments, orientation is achieved by H-PDLC formation under an orienting force. The display may further include an oriented or polar molecule to promote orientation of the LC droplets. The polar or orienting molecule may be and azo dye, and the azo dye is selected from the group consisting of Congo red, azobenzene, methyl orange, methylenene blue and crystal violet.
In another aspect of the invention, a polarizing light filter is provided having an holographically-formed polymer dispersed liquid crystal (H-PDLC) film comprising layers of liquid crystal (LC) droplets in a matrix polymer, the H-PDLC film having a reflection grating capable of reflecting light of a selected wavelength, wherein the reflection grating of the H-PDLC film is oriented, such that the refractive index parallel to the axis of orientation (ne) is greater than the refractive index perpendicular to the axis (no).
In yet another aspect of the invention, a method of preparing an holographic polymer dispersed liquid crystal (H-PDLC) film having bipolar oriented LC droplets is provided. A film comprised of a mixture of liquid crystal, a polar molecule and a photo-polymerizable monomer is illuminated with light of an energy sufficient to orient the polar molecule but insufficient to initiate polymerization of the photo-polymerizable monomer. The film is then illuminated with at least one holographic light pattern to obtain an holographically-formed polymer dispersed liquid crystal (H-PDLC) film comprising layers of liquid crystal (LC) droplets in a matrix polymer wherein the LC droplets contain bipolar LC molecules.
By xe2x80x9csecuring meansxe2x80x9d as that term is used herein is meant any clamp, fastener, support, glue, adhesive, cement, seal or paste, and the like which may be used to secure the strain gauge and the surface. The securing means may be a mechanical means, such as a clamp, clip or other suitable fastener. The mechanical securing means desirably secures the strain gauge to the surface in a manner which permits transmission of the strain experienced by the surface to the strain gauge. Exemplary means include bands or other clamps over an area or region of the H-PDLC (as compared to a point contact). The securing means also includes adhesives, such as glues and cements, that form a bond between the strain gauge and the surface. The bond can be established between the entire strain gauge or only a portion of the strain gauge. The bond transmits strain from the surface to the strain gauge.
By xe2x80x9cstrain,xe2x80x9d as that term is used herein is meant a deformation in a material resulting from an applied force or stress. In the strain gauge, that force is transmitted to it from the workpiece. In the workpiece that force is a tensile, compressive, torsional, or shearing force or the like, for which observation or detection by the gauge is desired.
By xe2x80x9creflection grating,xe2x80x9d as that term is used herein is meant a periodic array of LC droplet planes having an orientation and layer spacing sufficient to reflect light of a selected wavelength that is incident on the surface of the grating.
By xe2x80x9ctransmission grating,xe2x80x9d as that term is used herein is meant a periodic array of LC droplet planes having an orientation and layer spacing sufficient to diffract light and to transmit light of a selected wavelength.
The H-PDLC strain gauge of the invention has the advantage over mechanical or electrical transducer strain gauges in that it is not disturbed by electromagnetic interference. This is particularly desirable for making measurements in electromagnetically noisy environments where strong electrical and/or magnetic fields may be present. A further advantage of the strain gauge is its ability to provide information regarding the location and directionality of applied stress. The strain gauge may be used to detect material failures, or to monitor deformation over time to preempt failure. A preferred embodiment uses the strain gauge to detect weld deformation.